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Here is my scenario, we own a 2016 S75D ( wife's car) I am getting a 2014 S85 cpo with dual chargers in a few weeks.
My charging Plan was and still is to have my wife and I share the HWPC (setup is 240/50 amp breaker) in our garage. She drives 40-50 miles a day. I will drive 150-200/day average. She will charge as soon as she gets home, I will take over the HWPC at bedtime and charge over night. So I am concerned I could need more time to charge to full (90%) and now the dual chargers on the cpo could possibly guarantee me I would be fine for daily 90% over night charging.

1.) So after reading a lot today on the forum am I correct in understanding all I need to do is have an electrician install a 80 amp breaker to the HWPC and rest the unit to an 80 amp setting? The cars figure out the rest on there own?
2.) Would a 90% dual charge on the S85 take 4ish hours? 50 miles an hr average?
3.) What is the downside to doing this if any? I read some set the car to pull 60amps not 80amps? I am not sure why? Read it's hard on the dual chargers day after day?
Am I missing anything else?
Thanks all, I always appreciate your advice,
No ice

It seems like the 250ish total miles per night you need is probably doable from the current 240/50 connection, if you added a second HPWC slaved to the first one and sharing the circuit (this isn't any better than the swapping you had planned, except that it avoids the hassles and any chance of forgetting, and leaves both cars plugged in for preconditioning and vampire drain.)

Upgrading to a faster circuit would be better, of course.

(The discussion of circuit sharing assumes it's a 2016 or newer HPWC - older ones have different internals and can't share a line.)

There's nothing wrong with charging the car at 80A every night - that's what it was designed and rated for.

However, if there's no overarching reason to hurry, dropping the current to 64A instead decreases the heating to 64% of the 80A setting, which presumably means a longer life for the charger module (thermal cycling is likely the main source of wear and tear for the charger.)

I'm in the same boat (2 Model S's) - I installed a 200 AMP Sub panel in the garage and went with two 100 AMP circuits to the HPWC's, my wife's car only has a single charger, but my car has duals. Her charger is outside our garage and we have it on plug-share - so folks with dual chargers can get the full throughput. We have the old school HPWC's not the new daisy chained managed ones... it has worked well.

As someone already stated - whatever circuit you put in - only 80% can be used sustained based on US Codes and equipment thresholds.

50AMPS gets you 40 AMPS to the car (on a good day), 100AMPS gets you 80AMPS to the car (on a good day).

Also with two cars you want redundancy in case of charging equipment failure. Screw going outside to switch cables - pay the $600 and get a second charger (you get to write off the install anyway)

@OP, do what @andrewket & @NikeWings suggest. Get a 2nd HPWC.
Even with a single 100A circuit and both cars charging at 40A, 8 hours will be enough to get them both from 0-100% over night.
When we ordered our MX back in 2015, I added 50% more solar, and added a 2nd HPWC, knowing that sharing one would not be feasible. For under $1000, it will make life 100's of times easier.

I have the first gen HPWC that my wife uses for her X and I got a NEMA 14-50 outlet installed to charge my S with a UMC. Might be cheaper to get a 14-50 outlet compared to upgrading the panel to pull 200A and buying a 2nd HPWC. Your wife can use the UMC/14-50 outlet while you use the HPWC. The 14-50 outlet also allows other types of EV to charge as well.

We have an identical setup, one S with dual (80A) charging and a newer one with 48A. Went with one HPWC on 100A circuit for the 48A car and NEMA-14-50 on 50A circuit for the 80A car (so it charges at 40A). Between the 2 cars they draw 88A which my 200A house service can comfortably handle (our stove and dryer are gas, all lighting is LED). I even swapped the 2 cars around on one occasion and while the cars were feasting on 120A total no breaker tripped, but it was winter so no AC load which I suspect could eventually trip the main breaker. I considered the 2 HPWC setup but already had the first generation HPWC which does not support current sharing so would have to replace it and even then I could not be charging any faster due to service limitations. The current setup of 40+48 works great for us, and I have the option to do 40+80 if really needed, though I never needed it (did it the one time just out of curiosity). If I did 200 miles a day though, I would just put 2 HPWC's on 100A circuit and let the high mileage car sip 80A when possible.

For your setup @No ICE, if your house electrical service can handle it, just add another independent HPWC on a 100A circuit. There is a good chance this may be a bit too much unless you have 300A or 400A service though. Second option, second HPWC on a 100A circuit but have it talk to the first one. AFAIK they don't actually have to be on the same breaker since the communications between them is via a separate wire anyways - this will make sure that they will not draw more then 80A combined total. Another alternative, just add a NEMA 14-50 and use mobile connector - this is cheapest but will not let you utilize higher charging rate once one car is done charging.

1.) So after reading a lot today on the forum am I correct in understanding all I need to do is have an electrician install a 80 amp breaker to the HWPC and rest the unit to an 80 amp setting? The cars figure out the rest on there own?

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You need a 100A breaker to draw 80A. In most places in North Americal the code calls for max 80% sustained usage, and an EV is definitely sustained usage as it can charge for hours at a time.

3.) What is the downside to doing this if any? I read some set the car to pull 60amps not 80amps? I am not sure why? Read it's hard on the dual chargers day after day?

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Some set it to 60A to not overload their house service. Dual chargers are literally that, two 40A chargers, so charging at 80A on dual chargers is no harder than 40A on a single charger. Some theorize whether it's any different for the battery, but the battery has an 8 year unlimited mileage warranty so I wouldn't worry, and if you do 200miles a day, you will welcome 80A charging as some days you may need to make that additional 40 miles trip to pick something up in the evening and really won't want to wait too long.

For an example of what a dual HPWC install can look like, see my sig. I future-prooofed installing two for upcoming Model X and 3 purchases. You really want to come home and plug in and forget about it.

If I were you, I would not rush into 2 chargers or upgraded circuits. I would plug the 40-50 mile per day car into 110 circuit and let the 150-200 mile car use the hpwc. Once a week or so you may need to top off the 40-50 mile car on hpwc. This will allow you to charge Both cars overnight without plug sharing when TOU costs are lowest.

After a few months of that, you can decide whether to install more amps.

This is the setup we use for our P85 and eGolf. Works great.

We tried plug sharing and it ain't good! I guarantee you will have days when you forget to switch and have an oh crap moment in the AM.

If I were you, I would not rush into 2 chargers or upgraded circuits. I would plug the 40-50 mile per day car into 110 circuit and let the 150-200 mile car use the hpwc. Once a week or so you may need to top off the 40-50 mile car on hpwc. This will allow you to charge Both cars overnight without plug sharing when TOU costs are lowest.

After a few months of that, you can decide whether to install more amps.

This is the setup we use for our P85 and eGolf. Works great.

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I wouldn't do it. It may work for the eGolf, but there is a reason people buy a Tesla - bigger battery. If you're going to keep a Tesla at eGolf charge levels (so able to do 50 miles per day) you might as well save the money and buy an eGolf, or a Leaf - even then I would recommend a 6+KW charger, not a 1KW 110V. 3 miles per hour charging is not something I would recommend for a car that does 50 miles per day. As a reference, it would take almost 4 days to charge the 75D from 0% to 100%.

Depends on your driving habits. oP says wife drives 40-50 miles per day. You get 4 miles per hour on clean 110 circuit. That's 48 miles in 12 hours. Why should OP spend thousands of $ on electricians when it may not be needed? I'm suggesting try for a while. Then install more amps if needed. How can you disagree with that?

Depends on your driving habits. oP says wife drives 40-50 miles per day. You get 4 miles per hour on clean 110 circuit. That's 48 miles in 12 hours. Why should OP spend thousands of $ on electricians when it may not be needed? I'm suggesting try for a while. Then install more amps if needed. How can you disagree with that?

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First, it sounds like someone really took advantage of you. Thousands of dollars to install a NEMA 14-50 in a garage is highway robbery. Hundreds, yes, depending on how far, but not thousands, unless someone is really taking you to the cleaners or you have a super low electrical service and require an upgrade for the whole house.

Second, you do not get 4 miles per hour on a standard 110V socket. You can get 3miles, but that's only if it's a dedicated circuit. Remember that charging is only 90% efficient. There there is an issue of sharing the circuit. If there is a garage fridge on it or almost anything else. 1-2 miles per hour maybe the most you can set it to. Then there is vampire drain, which can run you 3-6 miles per day, depending on temperatures (you loose it when parked at work as well as at home). So, in 12hrs, you can bank on 24 miles, you may get 40 if you're lucky, and that's rated miles. A little rain, cold weather, and that 40 miles turns into 30.

I would challenge you to try to find one person who has a Tesla and drives 50 miles every day and is happy with 110V charging.

First, it sounds like someone really took advantage of you. Thousands of dollars to install a NEMA 14-50 in a garage is highway robbery. Hundreds, yes, depending on how far, but not thousands, unless someone is really taking you to the cleaners or you have a super low electrical service and require an upgrade for the whole house.

Second, you do not get 4 miles per hour on a standard 110V socket. You can get 3miles, but that's only if it's a dedicated circuit. Remember that charging is only 90% efficient. There there is an issue of sharing the circuit. If there is a garage fridge on it or almost anything else. 1-2 miles per hour maybe the most you can set it to. Then there is vampire drain, which can run you 3-6 miles per day, depending on temperatures (you loose it when parked at work as well as at home). So, in 12hrs, you can bank on 24 miles, you may get 40 if you're lucky, and that's rated miles. A little rain, cold weather, and that 40 miles turns into 30.

I would challenge you to try to find one person who has a Tesla and drives 50 miles every day and is happy with 110V charging.

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Jeez, I suggest the guy try something that would save him some $ and you disagree and pick a fight. Back off. Most recommendations here were to install 100 amps, not 50. Garage geography, remaining panel slots... all can drive price up. Why not try NO additional cost before spending? You can get 4 miles per hour on 110. I've done it many times.

Thank you all so much for the words of wisdom. I never thought about getting a second HWPC. Yes I have the gen 2 model installed, so going to a 100 amp circuit and sharing sounds so much easier. I assume when my wife's car hits her charge level, my S85 would then take the bulk of the amps and my dual charging would kick in? Or if she was not at home and I was plugged in sole I would be getting all 80amps to myself? Does it do this from either HWPC or would they be set up at 40 and 80 or would they both be 80 and 80 and the cars figure out what the share? This set up sounds good to me!

...all I need to do is have an electrician install a 80 amp breaker to the HWPC...

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I'm sure your electrician will point this out, but you should realize that the wiring in your wall dictates the circuit breaker size, so you can't just install a larger one without checking the ampacity of the wiring. Any circuit breaker's primary function is to protect the wiring -- and thus prevent fires. It is not intended to protect the appliance connected to the circuit nor does the appliance dictate the size of the breaker.

Second option, second HPWC on a 100A circuit but have it talk to the first one. AFAIK they don't actually have to be on the same breaker since the communications between them is via a separate wire anyways - this will make sure that they will not draw more then 80A combined total.

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@whitex, I've been intrigued by this setup for my own use. Do you know if this would violate any codes? From a purely practical viewpoint it seems reasonable but I'm not entirely certain.

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